1. Technical Field
The present invention relates to a pressure sensor, and in particular to a pressure sensor that does not use oil as a pressure receiving medium and relates to a technique for reducing an error in detection of a pressure caused by a support part of a pressure sensitive element.
2. Related Art
Pressure sensors that use a piezoelectric resonator as a pressure sensitive element are known as a water pressure gauge, an air gauge, and a differential pressure gauge. The piezoelectric resonator is configured, for instance, such that an electrode pattern is formed on a planar piezoelectric substrate, and a detection axis is set to a direction of detecting a force. When pressure is applied in the direction of the detection axis, a resonance frequency of the piezoelectric resonator is varied so that the pressure is detected on the basis of the variation in the resonance frequency. JP-A-56-119519, JP-A-64-9331, and JP-A-2-228534, as first, second, and third examples, of the related art disclose a pressure sensor including a piezoelectric resonator as a pressure sensitive element. When a pressure is applied to bellows from a pressure input orifice, a force F corresponding to an effective area of the bellows is transmitted to the piezoelectric resonator as a compressive force or a tensile force through a force transmitting unit that has a pivot as a fulcrum (a flexible hinge). A stress corresponding to the force F is generated in the piezoelectric resonator and the stress causes the resonance frequency to be varied. The pressure sensor is adapted to measure a pressure by detecting variation in the resonance frequency appearing in the piezoelectric resonator.
An existing pressure sensor is described below with reference to the first example of the related art. FIGS. 8A and 8B are schematic views typically illustrating a structure of a related art pressure sensor. A pressure sensor 501 according to the related art shown in FIG. 8A includes a housing 504 having first and second pressure input orifices 502 and 503 that are arranged to face each other, and a force transmitting member 505 disposed inside the housing 504. The force transmitting member 505 is coupled to a first bellows 506 and a second bellows 507 such that one end of the force transmitting member 505 is sandwiched between one end of the first bellows 506 and one end of the second bellows 507. The other end of the first bellows 506 is coupled to the first pressure input orifice 502 so that a pressure to be measured can be introduced to the inside, and the other end of the second bellows 507 is coupled to the second pressure input orifice 503 so that an atmospheric air pressure can be introduced to the inside. In addition, a double-ended tuning fork resonator 509 serving as a pressure sensitive element is disposed between the other end of the force transmitting member 505 and an end of a substrate 508 at an opposite side from a pivot (fulcrum).
Here, regarding the pressure sensor, the bellows at a detection side is filled with a liquid so as to detect a pressure with high precision. In general, oil such as silicon oil having high viscosity is used as the liquid, in order to prevent bubbles from entering and accumulating inside the bellows or between the folds of the bellows.
Thus, the interior of the first bellows 506 is filled with oil 510 having high viscosity. In a case where an object for pressure measurement is a liquid, the oil 510 is brought into contact with the liquid via an opening 511 formed at the first pressure input orifice 502 to face with the liquid. Here, a diameter of the opening 511 is set so that the oil 510 does not leak out.
In the pressure sensor 501 having the above structure, when the pressure F is applied to the oil 510 stored in the first bellows 506 from the liquid as the object for pressure measurement, the pressure F is applied to the one end of the force transmitting member 505 via the first bellows 506. At the same time, the atmospheric pressure is applied to the second bellows 507 and a force equivalent to the atmospheric pressure is applied to the one end of the force transmitting member 505.
As a result, a force equivalent to a differential pressure is applied through the other end of the force transmitting member 505 to the double-ended tuning fork resonator 509 as a compressive force or a tensile force with a pivot of the substrate 508 as a fulcrum point, the differential pressure being a difference between the atmospheric pressure and the pressure F applied by the liquid as the pressure measurement object. Due to the compressive force or the tensile force applied to the double-ended tuning fork resonator 509, a stress is generated in the resonator 509. In accordance with a magnitude of the stress, the resonance frequency of the resonator 509 is varied. Therefore, measurement of the resonance frequency enables detection of the magnitude of the pressure F.
JP-A-2005-121628, as a fourth example of the related art, discloses a pressure sensor having a structure that does not include an expensive force transmitting unit (cantilever) having a swing arm using a pivot (flexible hinge) as a fulcrum which is used in the above described pressure sensor. In the sensor, two bellows are straightly aligned in a sensor housing in a manner sandwiching a pedestal therebetween. The sensor detects pressure variation generated by an action of the pedestal caused by the difference between pressures applied to the respective bellows. Therefore, a resonator bonding pedestal is sandwiched between one end of the first bellows and one end of the second bellows. A pressure sensitive element is provided at an outer periphery side of the second bellows, and ends of the pressure sensitive element are fixed to the pedestal and to a housing wall positioned at the other end side of the second bellows. In addition, a reinforcing plate is disposed at an axisymmetrical position to the pressure sensitive element with the second bellows interposed. The ends of the reinforcing plate are fixed to the pedestal and the housing wall, respectively.
JP-A-2007-57395 as a fifth example of the related art discloses a pressure sensor including a reinforcing elastic member (spring) that couples a pedestal to a housing in a direction orthogonal to a direction of a pressure detection axis of bellows. The reinforcing elastic member is provided so as to solve such a problem that the sensor disclosed in the fourth example has insufficient strength with respect to a shock applied from a direction orthogonal to the direction of the pressure detection axis of the bellows.
JP-A-2006-194736 and JP-A-2007-132697 as sixth and seventh examples of the related art disclose a pressure sensor that is used by being fixed to an engine block so as to detect a hydraulic pressure inside an engine. The pressure sensor includes a sensing unit that outputs an electric signal corresponding to an applied pressure, a pressure-receiving diaphragm unit that receives a pressure, and a pressure transmitting member for transmitting the pressure from the diaphragm unit to the sensing unit. Specifically, a first diaphragm for reception of a pressure and a second diaphragm for detection are respectively provided to end faces of a hollow metal stem. A force transmitting member is interposed between the first diaphragm and the second diaphragm in the stem. The force transmitting member is a shaft made of metal or ceramic, and is interposed between the pair of diaphragms in a prestressed fashion. Further, a chip with a functionality of a strain gauge (strain gauge chip) is attached to an outer end face of the second diaphragm as a pressure detection element. The force transmitting member transmits a pressure received by the first diaphragm to the second diaphragm, and deformation of the second diaphragm is converted into an electronic signal by the strain gauge chip, thereby detecting the hydraulic pressure of the engine.
In the first to third examples, the first bellows 506 is filled with the oil 510 as the pressure sensor 501 shown in FIG. 8. The oil 510 has a thermal expansion coefficient higher than that of any other elements that constitute the pressure sensor 501, such as the force transmitting member 505 and the double-ended tuning fork resonator 509. As a result, thermal distortion occurs in the components constituting the pressure sensor due to a temperature change. Such thermal distortion acts on the double-ended tuning fork resonator 509 as unnecessary stress, resulting in inducing of an error in a measured pressure value and degradation of characteristics of the pressure sensor.
Moreover, since the oil 510 stored in the first bellows 506 contacts and faces a liquid that is an object for pressure measurement, the oil 510 may flow into the liquid, or the liquid may flow into the first bellows 506 depending on an installation way of the pressure sensor. This may cause bubbles to be generated inside the oil 510 stored in the first bellows 506. If bubbles are generated in the oil 510 serving as a pressure transmitting medium, a force cannot be stably transmitted to the double-ended tuning fork resonator 509 through the force transmitting member 505, possibly resulting in inducing of an error in a measured pressure value.
Further, as described above, since the oil 510 contacts and faces the liquid that is an object for pressure measurement, the oil 510 may flow into the liquid depending on an installation way of the pressure sensor. Therefore, an existing pressure sensor using the oil 510 according to the related art is not able to be applicable to measurement of a pressure of a pure liquid that dislikes foreign substances.
Furthermore, the pressure sensor 501 of the related art includes the force transmitting member 505 having a complicated structure, resulting in difficulties in miniaturizing of the pressure sensor. In addition, the force transmitting member 505 requires a flexible hinge having a slim constriction so as to be an expensive component, thereby disadvantageously increasing the manufacturing cost of the pressure sensor.
Furthermore, the pressure sensor 501 of the related art includes the cantilever type force transmitting member 505 having a complicated structure, resulting in difficulties in miniaturizing of the pressure sensor. In addition, the force transmitting member 505 requires a flexible hinge having a slim constriction so as to be an expensive component, thereby disadvantageously increasing the manufacturing cost of the pressure sensor.
When the pressure sensor of the fourth and fifth examples of the related art inclines, the bellows may droop. As a result, a force applied to the pressure sensitive element (double-ended tuning fork resonator) varies, resulting in variation in a resonance frequency.
In addition, the pressure sensor has a structure that one end of a pipe filled with oil is connected to a pressure introduction orifice of the pressure sensor and the other end of the pipe is brought into contact with a liquid that is an object for pressure measurement. As a result, the oil stored in the bellows or the pipe contacts and faces the liquid that is an object for pressure measurement, as described in the first to third examples.
Further, the pressure sensor has a structure that one end of a pipe filled with oil is connected to a pressure introduction orifice of the pressure sensor and the other end of the pipe is brought into contact with a liquid that is a measurement object. As a result, the oil stored in the bellows or the pipe contacts and faces the liquid that is an object for pressure measurement, as described in the first to third examples. Since the oil may flow into the liquid as the object for pressure measurement or the liquid may flow into the bellows depending on an installation way of the pressure sensor, bubbles may be generated in the oil stored in the bellows. If bubbles are generated in the oil, the oil functioning as a transmission medium of pressure cannot stably transmit a force through the pedestal to the double-ended tuning fork resonator, resulting in an error in the pressure measurement.
The pressure sensor of the fifth example of the related art has a structure that the pedestal sandwiched by the bellows is supported by the reinforcing elastic member made of a plate string provided at the side face of the housing. With the above configuration, it is possible to generate a force suppressing an action of the pedestal along with movement of the bellows in the direction of the axis. As a result, the pressure detection sensitivity may be deteriorated. If the reinforcing elastic member is hardened for its firm support, the movement of the bellows is suppressed, resulting in deterioration of the pressure detection sensitivity.
Further, in the fourth and fifth examples, since the reinforcing plate is disposed at an axisymmetrical position to the pressure sensitive element with the bellows interposed, the movement of the bellows is suppressed, resulting in deterioration of the pressure detection sensitivity.
In the sixth and seventh examples of the related art, the diaphragm and the shaft are in contact with each other in the prestressed fashion. The pressure sensor is used at a high temperature in a high pressure. With the above configuration, if the diaphragm and the shaft are rigidly fixed, the mechanism may be broken by the difference between thermal expansions of the components. In consideration of the thermal expansions, the diaphragm and the shaft are only in point contact with each other, and are not bonded by an adhesive. As a result, there is a very high possibility that the point contact position deviates when the diaphragm and the shaft operate by the pressure variation. If the point contact portion deviates, a force acting on both of the diaphragm and the shaft is dispersed, resulting degradation of precision in pressure detection. Moreover, in the sixth and seventh examples of the related art, the pressure sensor is generally used at a high temperature in a high pressure. Therefore, it is desirable that the force transmitting member be as long as possible by creating a distance between the pressure receiving unit and the sensing unit in order to avoid thermal influence to the components such as the chip of the sensing unit. Thus, the sensor described in the examples is not suitable for miniaturization. In addition, in the case of the sixth and seventh examples of the related art, transmission of a force is carried out by interposing a shaft between a pair of diaphragms. However, since the sensor chip is attached to the diaphragm in the sensing unit, the properties of the diaphragms respectively provided to the pressure receiving unit and the sensing unit differ from each other, resulting in a problem that the measurement accuracy cannot be improved.